1. Field of the Invention
The Present invention relates to a method and apparatus for cleaning a water basin floor. More particularly, the invention relates to an underwater basin cleaning vehicle which removes a clogging layer of debris and growth from the bottom floor of the basin, without removing substantial amounts of sand and/or gravel material residing underneath and mixed with the clogging layer.
2. Description of the Related Art
Many governmental organizations, or entities, own and operate water retrieval and purification systems, otherwise known as basins, for the purpose of supplying the water demands of a respective town, city or county. For example, the Orange County, California, Water District (OCWD) owns and operates seven basins within the Santa Ana River in Anaheim, Calif. These basins are between 10 and 60 feet deep with individual surface areas of between 11 and 71 acres, for a total of over 8,000 acre-feet of surface-storage capacity.
In Orange County, water from the Santa Ana River is conveyed into the basins through a system of pipelines, channels, and settling lagoons. The water in the basins filters through a bottom layer composed mostly of sand and gravel. The water "percolates" through the bottom sand layer and into an underlying aquifer where it is stored for subsequent consumption. Aquifers are large underground formations which are typically filled with porous gravel and rock materials. The water is stored in the "pores" of the aquifer from which it can be pumped to be retrieved and consumed. In Northern Orange County, California, for example, aquifers supply up to 75% of the drinking water needs for that region. Except during storm events when water is lost to the Pacific Ocean, the entire flow of the Santa Ana River is captured for the benefit of the community.
The natural sand and gravel at the bottom of the basins is coarse to very coarse, with a median grain size of 0.9 millimeters, or 90 microns. These coarse particles assist in the filtration of the basin water as it percolates through to the underlying aquifers. It has been observed that percolation rates in the basins drop dramatically after the basins have been in use for several weeks. Samples taken from the basins indicate that this is primarily due to the accumulation of fine sediment and biological growth on the basin floor. This fine sediment and biological growth which forms a "clogging layer," prevents the water in the basins from percolating through the underlying sand layer and into the underground aquifers. Therefore, percolation into the aquifer is impeded. As used herein, the terms "sediment," "sediment particles," "clogging layer," "fine particles" and any combinations or conjugations thereof are used synonymously and interchangeably, and refer to any debris, matter, substance, chemical, biological growth, or other material which may be found on the floor of a body of water and which typically exhibits smaller particle size than the natural sand found on the underwater floor.
Because the basins are above the water table, there are times when the soils below the basins are unsaturated. This condition exacerbates the bottom-clogging phenomenon because the pressure differential between the total head in the basin (20-40 psi) and the atmospheric pressure in the underlying soils (15 psi) tends to compress the intervening sediments. In the case of the clogging material, it is suspected that the pressure compresses the fine particles and the algae into a thin, dense, and relatively impermeable layer on the natural sandy basin floor.
Prior methods of cleaning this layer of clogging material from the basin floor include regularly draining the basins and mechanically scraping away and removing the clogging layer with earth-moving equipment, such as a bulldozer. This process temporarily increases the percolation rates for the basins. However, the clogging layer typically reforms within several weeks and the cleaning process must be repeated. This drain-and-scrape cleaning method requires substantial manpower and necessitates that the basins remain out of service for several days to weeks during the process.
Additionally, these prior methods of removing the clogging layer also removed some of the underlying sand layer which is vital to the natural filtration process of the water. Ideally, the scraped materials would consist of only the fine particles and biological growth which constitute the clogging layer, with little of the underlying natural sand. However, it is difficult to achieve this objective because it is difficult to remove all of the clogging material without removing a large portion of the underlying native sand. Furthermore, over time and with repeated dry-cleaning operations, the silt and clay tend to winnow downward as much as several feet beneath the bottom sand surface of the basin, detrimentally affecting the natural filtration process provided by the sand.
In view of the above-described problems, what is needed is a method and system for removing the clogging materials from the bottom surface of a basin without draining the basin, and without removing substantial amounts of the underlying sand which is needed to filter the water as it percolates through to the underlying aquifer.